The present invention relates to a light-emitting-diode array and a fabrication method thereof, more particularly to an inexpensive method of fabricating a high-density light-emitting-diode array.
A light-emitting diode (also referred to as a LED) is basically a pn junction. It is known technology to fabricate a light-emitting-diode array by diffusing a p-type impurity such as zinc through an diffusion mask having multiple diffusion windows into an n-type semiconducting substrate such as n-type gallium arsenide (GaAs). In conventional light-emitting-diode arrays, the underside of the substrate is coated with a metal film that serves as a common cathode electrode, and an individual metal anode electrode is provided for each light-emitting diode on the upper side. The anode electrode pattern for each light-emitting diode comprises a comparatively large bonding pad, formed on the diffusion mask, or on an inter-layer insulating film, and a narrower line connecting the bonding pad to the light-emitting diode. The bonding pads are coupled by wire bonding to a separate integrated circuit (IC) that drives the light-emitting-diode array. Arrays of this type with a single row of light-emitting diodes are used as light sources in electrophotographic printers.
With the electrode structure described above, however, the density of the light-emitting diodes is limited by the density with which the bonding pads can be laid out and the bonding wires attached. Even by placing the bonding pads on alternate sides of the row of light-emitting diodes, it is difficult to achieve densities as high as twelve hundred light-emitting diodes per inch (1200 dpi), or higher, which are desired densities for high-quality printing. Moreover, even if a light-emitting-diode array of this density could be fabricated, connecting the array to its driver ICs by wire bonding would present a difficult and perhaps insurmountable challenge.
To overcome this problem, Japanese Kokai Patent Publication No. 152873/1987 discloses a matrix driving scheme in which the light-emitting diodes are divided into groups, each group being formed in a separate n-type well in a p-type substrate. Each n-type well is coupled through an npn bipolar transistor to the common cathode electrode of the array. This arrangement reduces the number of bonding pads, but the bipolar transistors add considerably to the size, complexity, and fabrication cost of the array.
Japanese Kokai Patent Publication No. 177478/1988 discloses a matrix driving scheme in which each light-emitting diode is disposed in an individual mesa of semiconductor material that is electrically isolated from the substrate. This scheme causes planarization problems, which can lead to electrical discontinuities in electrode lines.
Japanese Kokai Utility Patent Publication No. 170142/1988 discloses a matrix scheme in which the light-emitting-diode array is divided into sections that are electrically isolated from one another by intervening layers of a dielectric material. This type of dielectric isolation requires extra fabrication process steps, and is not easily achieved at a low cost.